A transmission system is known which transparent-relays a Gigabit Ethernet signal (hereinafter referred to as a “GbE (Gigabit Ethernet®) signal”) via an OTN (Optical Transport Network) line that is different from the Gigabit Ethernet. That is, a transmission system which connects a plurality of lines of the Gigabit Ethernet (hereinafter referred to as “GbE lines”) via the OTN line is known. In the transmission system, a GbE signal is transmitted from a transmission apparatus of the Gigabit Ethernet (hereinafter referred to as a “GbE transmission apparatus”) via the GbE line to a transmission apparatus located at the connection point of the Ethernet line and the OTN line (hereinafter referred to as an “OTN transmission apparatus”). An OTN transmission apparatus transparently maps the GbE signal to an ODU signal of the OTN and transmits the ODU signal via the OTN line to another OTN transmission apparatus. That is, without caring about a MAC (Media Access Control) frame which would usually be referred to at transmission or reception of a GbE signal, the OTN transmission apparatus transmits the GbE signal (10B code) to another OTN transmission apparatus by using an ODU signal on the OTN line. The other OTN transmission apparatus transparently demaps the GbE signal from the received ODU signal and transmits it to another GbE transmission apparatus via the GbE line. That is, without caring about a MAC (Media Access Control) frame which would usually be referred to at transmission or reception of a GbE signal, the other OTN transmission apparatus transmits, to another GbE transmission apparatus, the GbE signal (10B code) transferred using the ODU signal on the OTN line. In this way, GbE signals are transparently transmitted and received between the two GbE transmission apparatuses via the OTN line.
As described above, in the transmission system which relays a GbE signal via the OTN line, when a fault occurs in the OTN line, it becomes impossible to normally relay the GbE signal via the OTN line or it becomes difficult to do so. Alternatively, when a fault occurs in a GbE line on the upstream side of the OTN line (i.e., the transmission side of the GbE signal as seen from the OTN line), it becomes impossible to normally relay the GbE signal via the OTN line to a GbE line on the downstream side of the OTN line (i.e., the reception side of the GbE signal as seen from the OTN line) or it becomes difficult to do so. In other words, when a fault occurs in a communication performed through the OTN line, it becomes difficult to maintain a normal communication between a GbE transmission apparatus on the upstream side of the OTN line and a GbE transmission apparatus on the downstream side of the OTN line. Accordingly, when a fault occurs in a communication performed through the OTN line, it is preferable that the occurrence of a fault be reported to the GbE transmission apparatus. In the meantime, a digital wrapper is used as a frame format technology for an optical transmission network, and it is applied to a communication which uses an OTN line. In the meantime, a digital wrapper is also incorporated into ITU-T G.709.
As a technique for reporting an occurrence of a fault, a technique is possible in which the occurrence is reported using an auto negotiation function conforming to IEEE802.3. More specifically, a technique is possible in which, when a fault occurs in a communication performed through an OTN line, the occurrence of a fault is reported to an Ethernet transmission apparatus by transmitting an auto negotiation restart signal (Link Fault) from an OTN transmission apparatus to a GbE transmission apparatus.